Wednesday, June 20, 2012

A Vinyl Window with a Great R-Value

Plastics News is reporting on a new window with an R-14 insulation value. As you would expect, the window is of vinyl construction (Plastics News isn't going to report on an all-wood window now are they?). An R-14 rating for a window is pretty high while most other windows are in the 1-3 range. This is still quite short of the R-value for insulated walls which are in the 20 range.

The problem with just looking at the R-values however, is that they influence the overall insulative properties of a wall in a non-linear fashion. For example, let's look at a R-2 window that takes 10 % of an wall that is otherwise R-20. The R-value for the wall is NOT R = A1R1 + A2R2 = (0.9 x 20) + (0.1 x 2) = 18.2, where the A's are the relative areas of the wall/window and the R's are the R-values. Instead, the correct way to calculate the overall R-value is to consider the overall resistance that arises from parallel resistances – just like we all learned in elementary circuits. Heat escaping through the wall is taking a path parallel to that escaping through the window. This means that 1/R = A1/R1 + A2/R2 = 0.9/20 +0.1/2 = 0.045 + 0.05 = 0.095, so that R = 1/0.095 = 10.5. An R-value of 10.5 for a wall is nowhere near the 18.2 calculated earlier. You can also see in this case that the even though the area of the window is just 10% of the wall, the heat loss though it is the same as through the wall, and a window that is just 10% of the wall is pretty small. If the wall is 8 ft x 10 ft, then the window would be only about 10 in x 12 in, something like the window on an airplane.

If we now substitute the R-2 window with an R-14 window, the R-value of the wall rises to 19. Alternatively, we can increase the size of the window to 30% of the wall (30 in x 36in) and still have an R-value of 17.7. That's why an R-14 window is exciting news.